1. Field of the Invention
The present invention relates to a center axis offset measuring device which is used in a single-crystal production apparatus based upon Czochralski method and which is capable of measuring any offset of the center axis of a crystal lifting wire with respect to the axis of rotation of a crucible.
2. Description of the Related Art
In general, an apparatus for producing a single crystal based upon Czochralski method employs a wire or a shaft for lifting a grown single crystal. In order to reduce the overall height of the apparatus, a wire is preferably used rather than a shaft. The crystal lifting wire is suspended from an upper portion of the apparatus into a rotary crucible containing a molten material of the crystal. If the center axis of the crystal lifting wire is offset from the axis of rotation of the crucible, unbalance is caused in motion of the growing crystal that it cannot stationarily rotate around its axis at a special point of the molten material surface in the crucible. Thus, crystal lifting operation cannot be continued when the unbalance becomes large.
Offset of the center axis of the crystal lifting wire from the axis of rotation of the crucible inevitably occurs due to, for example, a mechanical distortion of the crystal production apparatus during a long use thereof, even though the lifting wire is initially set in axial alignment with the crucible, when a newly installed one of the apparatus is issued.
It is therefore necessary that a periodical readjustment has to be conducted so as to reduce this offset to zero during use of the single crystal production apparatus, not to mention the adjustment at the time of the initial setting.
Unfortunately, no specific consideration has been given to this problem, and the readjustment has been conducted with the help of a naked eye without conducting special measurement of the offset amount, so that an impractically long time is required for the centering of the lifting wire and the accuracy of the centering was not expected to be so high. Thus, the conventional single crystal production method tends to suffer from the above-described problem due to the offset.
This problem undesirably lowers the yield of good single crystal product. In addition, this problem becomes more serious as the diameter of the single crystal to be lifted increases. Thus, the present inventors have recognized this problem should be strictly overcome from the view point of reduction in the production cost.